Quantum cascade laser with monolithically integrated passive waveguide

ABSTRACT

A photonic integrated circuit device includes a passive waveguide section formed over a substrate, a quantum cascade laser (QCL) gain section formed over the substrate and adjacent to the passive waveguide section, and a taper section disposed between and in contact with each of the passive waveguide section and the QCL gain section. In some embodiments, the passive waveguide section includes a passive waveguide core layer disposed between a first cladding layer and a second cladding layer. In some examples, the QCL gain section includes a QCL active region disposed between a first confinement layer and a second confinement layer, where the QCL active region has a lower index of refraction than each of the first and second confinement layers. In some embodiments, the taper section is configured to optically couple the QCL gain section to the passive waveguide section.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/827,568, filed Apr. 1, 2019, the entirety of which is incorporated byreference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under DE-SC0012575awarded by the Department of Energy. The government has certain rightsin the invention.

BACKGROUND

Mid-infrared (mid-IR, λ≈3-30 μm) photonic integrated circuits (PICs) onlow-loss passive waveguide platforms are of great interest for a widerange of mid-infrared applications, including chemical sensing, powercombining, beam steering, and frequency conversion. Such devices maytransform many traditional mid-IR free-space instruments based ondiscrete optical components into compact, mass-producible, chip-scaledevices where all active and passive devices are combined onto a singlechip. With respect to mid-IR light sources, quantum cascade lasers(QCLs) are currently the only electrically-pumped semiconductor lightsources that can provide continuous-wave (CW) operation at roomtemperature in most of the mid-IR spectral range and, with cooling,across the entire mid-IR spectral range.

Thus far, monolithic integration of mid-IR QCLs with passive dielectricwaveguides has been demonstrated by heterogeneous integration of mid-IRQCLs on Si-based photonic platforms. For example, QCLs have beenintegrated onto a silicon-on-nitride-on-insulator wafer, having passivedielectric waveguides define therein, by direct wafer bonding (e.g., byflip-chip bonding). Heterogeneous integration of mid-IR QCLs onsilicon-on-sapphire wafers has also be reported based on atransfer-printing technique. Although monolithic integration of QCLsonto silicon-based waveguiding platforms offers some advantages such ascompatibility with silicon foundries for processing, such deviceplatforms present inherent thermal extraction and thermal stress issuesdue to the presence of a physical bonding interface, which is expectedto limit their use to low power applications. Efficient thermalextraction is especially critical for QCLs, which typically dissipatemore than one order of magnitude higher power per unit area compared todiode lasers. Moreover, given the very high thermal dissipation in QCLactive regions, achieving long-term reliability and CW operation ofheterogeneously-integrated devices on silicon platforms is challenging.Further, the use of the Si-based platforms limits the spectral range ofmid-IR PICs to below 7 μm due to optical absorption in silicon.

Thus, existing techniques have not proved entirely satisfactory in allrespects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a plot showing calculated optical loss as a function ofwavelength in various semiconductor materials for a plurality ofelectron concentrations, in accordance with some embodiments;

FIG. 1B is a plot showing resistivity and resistance as a function ofelectron concentration for various semiconductor materials, inaccordance with some embodiments;

FIG. 2A shows a top-down view of an exemplary mid-infrared (mid-IR)photonic integrated circuit (PIC) device, in accordance with someembodiments;

FIG. 2B shows a cross-sectional view along a section A-A′ of theexemplary mid-IR PIC device of FIG. 2A, according to some embodiments;

FIG. 3 shows a cross-sectional view along a section B-B′ of theexemplary mid-IR PIC device of FIG. 2A and illustrating first and secondconfinement layers, according to some embodiments;

FIG. 4 shows a cross-sectional view of another exemplary mid-IR PICdevice including a current injection layer, according to someembodiments;

FIG. 5A illustrates a plot of a confinement factor for the fundamentalmode in the QCL active region and in the passive waveguide core as afunction of passive waveguide core thickness, in accordance with someembodiments;

FIGS. 5B, 5C, and 5D illustrate cross-sectional profiles (along asection C-C′ of FIG. 2B) of a simulated fundamental mode for a pluralityof passive waveguide core thicknesses, in accordance with someembodiments;

FIGS. 6A and 7A provide cross-sectional views of devices without andwith, respectively, upper and lower confinement layers, along a sectionsubstantially similar to section B-B′ of FIG. 2A, according to someembodiments;

FIG. 6B shows a cross-sectional profile of the simulated modeconfinement, corresponding to the device of FIG. 6A, within the QCL gainsection and along a section substantially similar to the section C-C′ ofFIG. 2B, in accordance with some embodiments;

FIG. 7B shows a cross-sectional profile of the simulated modeconfinement, corresponding to the device of FIG. 7A, within the QCL gainsection and along a section substantially similar to the section C-C′ ofFIG. 2B, according to some embodiments;

FIG. 8 illustrates a plot showing effective refractive indices of QCLwaveguide modes and the passive waveguide modes as a function of ridgewidth, in accordance with some embodiments;

FIG. 9A provides simulation results for a transverse mode transition ofa TM₀₀ QCL mode to a TM₀₀ waveguide mode along the section A-A′ shown inFIG. 2A, according to some embodiments;

FIGS. 9B, 9C, and 9D respectively show cross-sectional profiles of thesimulated mode confinement at each of a QCL gain section, a tapersection, and a passive waveguide section, according to some embodiments;

FIG. 10A illustrates a device biased according to a top-side-bottom biasconfiguration, according to some embodiments;

FIG. 10B illustrates a device biased according to a top-bottom biasconfiguration, in accordance with some embodiments;

FIGS. 11A, 11B, and 11C illustrate measurement results for a referencedevice and a waveguide-coupled device, according to some embodiments;

FIG. 12 illustrates a plot showing the peak output optical powercollected from a device having a wide QCL gain section and ahigh-reflectivity coating applied to the QCL facet, in accordance withsome embodiments;

FIG. 13A shows a top-down view of an exemplary buried-heterostructure(BH) mid-IR PIC device, in accordance with some embodiments; and

FIG. 13B shows a cross-sectional view along a section F-F′ of theexemplary BH mid-IR PIC device of FIG. 13A, according to someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Quantum cascade lasers (QCLs) are presently the only electrically-pumpedsemiconductor light sources that can provide continuous-wave (CW)operation at room temperature across most of the mid-IR spectral rangeand, with cooling, across the entire mid-IR spectral range. As such,monolithic integration of mid-IR (λ≈3-30 μm) QCLs onto low-loss passivedielectric waveguide platforms are of great interest for the developmentof mid-IR photonic integrated circuits (PICs) that can provide compact,mass-producible, chip-scale devices where all active and passive devicesare combined onto a single chip. Monolithic integration of mid-IR QCLswith passive dielectric waveguides has so far been demonstrated byheterogeneous integration of mid-IR QCLs on Si-based photonic platforms.

For example, QCLs have been integrated onto asilicon-on-nitride-on-insulator wafer by direct wafer bonding.Heterogeneous integration of mid-IR QCLs on silicon-on-sapphire wafershas also be reported based on a transfer-printing technique. However,such integration of QCLs onto Si-based platforms presents inherentthermal extraction and thermal stress issues due to the presence of aphysical bonding interface. This is expected to limit the use of QCLs onSi-based platforms to low power applications. Efficient thermalextraction is especially critical for QCLs, which typically dissipatemore than one order of magnitude higher power density compared to diodelasers. Moreover, given the very high thermal dissipation in QCL activeregions, achieving long-term reliability and CW operation ofheterogeneously-integrated devices on silicon platforms is quitechallenging. Thus, existing techniques have not proved entirelysatisfactory in all respects.

Embodiments of the present disclosure offer advantages over the existingart, though it is understood that other embodiments may offer differentadvantages, not all advantages are necessarily discussed herein, and noparticular advantage is required for all embodiments. For example,embodiments discussed herein include mid-IR PICs providing homogeneousmonolithic integration of mid-IR QCLs with a low-loss passive waveguideheterostructure that is grown together with a QCL heterostructure on aIII-V-based platform, such as an InP-based platform. In such anapproach, a mid-IR QCL heterostructure and a low-loss passive waveguideheterostructure (e.g., such as a low-loss passive InGaAs waveguideheterostructure) are formed together on a single substrate (e.g., suchas an InP substrate), thereby mitigating challenges associated withheterogeneous integration of mid-IR QCLs on Si-based photonic platforms.Moreover, and as a result of employing homogenous monolithicintegration, the layer growth and other processing steps may becompatible with those used for conventional high-performance mid-IRQCLs. In addition, unlike InP-based diode lasers in the near IR range,photon energy in the mid-IR band is several times smaller than band gapenergies of InP-based material systems (InGaAs, InAlAs, etc.) used inmid-IR QCLs. As a result, III-V mid-IR PICs, such as those describedherein, can avoid the problems of multi-photon absorption that arepresent in near-infrared III-V PICs.

The disclosed mid-IR PICs also provide for room temperature mid-IR QCLoperation and coupling to passive waveguides (e.g., such as passiveInGaAs waveguides) grown monolithically on the same wafer (e.g., thesame InP wafer). In accordance with some embodiments, the mid-IR PICsdisclosed herein include a QCL gain section, a taper section, and apassive waveguide section (e.g., such as an InGaAs passive waveguidesection). Lasing operation and coupling of a QCL mode into a passivewaveguide mode may be achieved by designing a QCL waveguide thatconfines the laser mode in the QCL active region of the QCL gain sectionand employing an adiabatic taper within the taper section to couple theQCL mode to the passive waveguide mode, as described in more detailbelow. In various examples, a lower confinement layer and an upperconfinement layer may also be formed to provide high refractive indexlayers above and below the QCL active region of the QCL gain section,which improve laser mode confinement and provide for efficient lasingoperation. In some embodiments, the taper section includes a two-stepadiabatic taper of a QCL heterostructure that is used to monolithicallycouple QCL light into the passive waveguide. In some cases, the tapersection also includes an adiabatic taper of a passive waveguideheterostructure. In one example, over 400 mW of mid-IR optical power atλ≈4.6 μm may be coupled into a 5-μm-wide passive waveguide with a750-nm-thick undoped In_(0.53)Ga_(0.47)As core surrounded by undoped InPcladding layers. For purposes of this disclosure, the term “undoped” maybe used to describe a semiconductor layer or region that is notpurposefully doped (e.g., by an ion implantation process, diffusionprocess, or other doping process). In an embodiment, the discloseddevices may provide an output power of 290 mW measured from the passivewaveguide facet at room temperature. Such an output power is nearly anorder of magnitude higher compared to the best results obtained withheterogeneously-integrated QCLs on Si at a similar operation wavelength.Generally, the embodiments disclosed herein represent a critical steptowards the development of mid-IR QCLs PICs demanding continuous-waveand high-power operation for practical applications such as chemicalsensing, power combining, and beam steering. Other embodiments andadvantages will be evident to those skilled in the art upon reading thepresent disclosure.

In various embodiments, the monolithic integration of mid-IR QCLs asdisclosed herein includes the use of In_(0.53)Ga_(0.47)As/InP materialsystems for the passive waveguide and cladding layers. IntrinsicIn_(0.53)Ga_(0.47)As and InP materials are nearly transparent in themid-wave-IR (MWIR, λ≈3-5 μm) and long-wave-IR (λ≈5-12 μm) ranges. Thus,the actual loss in waveguides made of In_(0.53)Ga_(0.47)As and InPmaterials may be principally determined by free carrier loss due tounintentional doping, as well as by fabrication imperfections such aswaveguide sidewall roughness. In some examples, a background dopinglevel of undoped InP and In_(0.53)Ga_(0.47)As may be in the range of1-5×10¹⁵ cm⁻³. Therefore, in some cases, an “undoped” semiconductorlayer may include a layer having a doping concentration in the range of1-5×10¹⁵ cm⁻³.

It is also noted that for the III-V material systems described herein,and in some embodiments, the dopant species used to dope one or more ofthe disclosed III-V material layers may include Si. To be sure, in someembodiments, other dopants, such as sulfur, selenium, or tin, mayequally be used without departing from the scope of the presentdisclosure. For most operating regimes of the disclosed mid-IR PICs, itmay be assumed that substantially all dopants introduced into the one ormore III-V material layers of the disclosed devices are fully ionizedand contribute to providing free carriers. As such, doping concentrationof a particular material layer may be substantially the same as freeelectron concentration. Thus, for purposes of this disclosure, the terms“doping level”, “doping concentration”, “carrier concentration”,“electron concentration”, and “free electron concentration” may at timesbe used interchangeably throughout the discussion that follows.

With reference now to FIG. 1A, illustrated therein is a plot 100 showingcalculated optical loss (dB/cm) as a function of wavelength (μm) inIn_(0.53)Ga_(0.47)As (solid curves) and InP (dashed curves) materialsfor electron concentrations (indicated using dashed ovals) of 1×10¹⁴cm⁻³, 1×10¹⁵ cm⁻³, and 5×10¹⁵ cm⁻³, in accordance with some embodiments.The data shown in the plot 100 indicates that propagation losses below 1dB/cm (absorption coefficient below 0.2 cm⁻¹) may be achieved inIn_(0.53)Ga_(0.47)As/InP passive waveguides for wavelength rangesbetween 3 μm and 12 μm, assuming a free electron concentration of about1×10¹⁴ cm⁻³. Such a loss is comparable to results achieved insilicon-on-insulator (SOI) and silicon-on-sapphire (SOS) platforms inthe MWIR region (e.g., 0.6 dB/cm at 3.4 μm in SOI and 0.74 dB/cm at 4.5μm in SOS).

In the MWIR range (λ≈3-5 μm), as shown in the plot 100, the calculatedoptical loss in In_(0.53)Ga_(0.47)As and InP remains well below 1.5dB/cm even if the free electron concentration is as high as 5×10¹⁵ cm⁻³.Such a doping level enables retention of sufficient electricalconductivity in the passive waveguide layer, as illustrated in the plot150 of FIG. 1B (which shows resistivity and resistance as a function ofelectron concentration), to extract current from the QCL active regionthrough the passive layers. Experimental data, which demonstrates suchdevice properties, is shown and discussed in more detail below. Thus, invarious embodiments and in contrast to QCLs on Si which may requirelateral current injection, III-V mid-IR QCL PIC devices (disclosedherein), may be processed in substantially the same manner as regularhigh-power mid-IR QCLs. It is noted that the resistance data shown inthe plot 150 includes in-series resistance data for a 1 μm thick layerof InP and In_(0.53)Ga_(0.4 7)As materials integrated beneath a QCLactive region. Further, the data shown in the plot 150 is calculatedassuming a QCL ridge waveguide having a width equal to about 6 μm and alength equal to about 4 mm.

Referring to FIGS. 2A and 2B, illustrated therein is an exemplary mid-IRPIC device 200, in accordance with some embodiments. In particular, FIG.2A shows a top-down view of the device 200 and FIG. 2B provides across-sectional view along a section A-A′ shown in FIG. 2A. In variousembodiments, the device 200 includes a QCL gain section 202, a tapersection 204, and a passive waveguide (WG) section 206 (e.g., such as anIn_(0.53)Ga_(0.47)As passive waveguide section). By way of example, aQCL heterostructure of the device 200 includes a first QCL ridge width‘W1 _(QCL)’. In various embodiments, the first QCL ridge width ‘W1_(QCL)’ narrows (within the taper section 204) to a second QCL ridgewidth ‘W2 _(QCL)’ along a first taper step 203 and to a third QCL ridgewidth ‘W3 _(QCL)’ along a second taper step 205. In other embodiments,narrowing of the first QCL ridge width ‘W1 _(QCL)’ to the third QCLridge width ‘W3 _(QCL)’ (within the taper section 204) may beaccomplished by using a single taper step or by using more than twotaper steps. The taper section 204 may also define a taper length ‘L’.It is noted that the QCL heterostructure spans the QCL gain section 202and the taper section 204 but is absent from the passive waveguidesection 206. In addition, a passive waveguide heterostructures of thedevice 200 includes a first passive waveguide ridge width ‘W1 _(PW)’ anda second passive waveguide ridge width ‘W2 _(PW)’, where the firstpassive waveguide ridge width ‘W1 _(PW)’ narrows (within the tapersection 204) to the second passive waveguide ridge width ‘W2 _(PW)’along a taper step 207. In some embodiments, narrowing of the firstpassive waveguide ridge width ‘W1 _(PW)’ to the second passive waveguideridge width ‘W2 _(PW)’ (within the taper section 204) may alternativelybe accomplished by using two or more taper steps. In variousembodiments, the passive waveguide heterostructure spans the QCL gainsection 202, the taper section 204, and the passive waveguide section206. In at least some examples, tapering of both the QCL heterostructureand the passive waveguide heterostructure may be performed to improvecoupling efficiency, for example, by increasing an interaction lengthalong which effective indices of a QCL mode and a passive waveguide modematch (as discussed below with reference to FIG. 8) within the tapersection 204. Also, it is noted that the taper section 204 issubstantially aligned to the QCL gain section 202 and to the passivewaveguide section 206 (e.g., along section A-A′ of FIG. 2A).

As shown, the device 200 includes a substrate 208. In some embodiments,the substrate 208 includes a doped InP substrate (e.g., InP doped withsulfur with a concentration between about ×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³),upon which various epitaxial QCL and passive waveguide layers (discussedbelow) are formed using a metal-organic vapor phase epitaxy (MOVPE)process, a metal-organic chemical vapor deposition (MOCVD) process, amolecular beam epitaxy (MBE) process, or other suitable epitaxialsemiconductor growth process. In at least one embodiment, the substrate208 may be doped to a concentration of about 1×10¹⁷ cm⁻³ and may have athickness of about 350 μm.

More generally, and in some embodiments, the substrate 208 may be asemiconductor substrate including another type of semiconductor such asGaAs, GaP, GaSb, InAs, InSb, or other appropriate substrate. Thesubstrate 208 may include various layers, including conductive orinsulating layers formed on the substrate 208. The substrate 208 mayinclude various doping configurations depending on design requirementsas is known in the art. For example, different doping profiles (e.g.,n-type or p-type doping) may be formed on the substrate 208 in regionsdesigned for different device types. The different doping profiles mayinclude ion implantation of dopants and/or diffusion processes. In someembodiments, the substrate 208 includes isolation features (e.g.,regions with low or substantially no current conductivity) interposingthe regions providing different device types. Further, the substrate 208may optionally include one or more epitaxial layers, may be strained forperformance enhancement, may include a silicon-on-insulator (SOI)structure, and/or have other suitable enhancement features.

In various embodiments, a passive waveguide heterostructure 210 isinitially grown over the substrate 208. The passive waveguideheterostructure 210 may include a lower passive waveguide cladding layer212 formed over the substrate 208, a passive waveguide core 214 formedover the lower passive waveguide cladding layer 212, and an upperpassive waveguide cladding layer 216 formed over the passive waveguidecore 214. In some embodiments, the lower passive waveguide claddinglayer 212 includes an undoped InP passive lower cladding layer. In atleast one example, the lower passive waveguide cladding layer 212 has athickness of about 3000 nm and a refractive index of about 3.08. In someembodiments, the lower passive waveguide cladding layer 212 has athickness in a range of about 2000-4000 nm. The passive waveguide core214 may include an In_(0.53)Ga_(0.47)As passive waveguide core. In anexample, the passive waveguide core 214 has a thickness of about 750 nmand a refractive index of about 3.38. In some embodiments, the passivewaveguide core 214 has a thickness in a range of about 500-900 nm. Insome cases, the upper passive waveguide cladding layer 216 may includean undoped InP passive upper cladding layer. In one example, the upperpassive waveguide cladding layer 216 has a thickness of about 1500 nmand a refractive index of about 3.08. In some embodiments, the upperpassive waveguide cladding layer 216 has a thickness in a range of about1000-1500 nm. Thus, in some examples, the lower passive waveguidecladding layer 212 and the upper passive waveguide cladding layer 216may have substantially the same refractive index, and a lower refractiveindex than the passive waveguide core 214. Stated another way, thepassive waveguide core 214 has a first volume-averaged refractive index,the upper passive waveguide cladding layer 216 has a secondvolume-averaged refractive index, and the lower passive waveguidecladding layer 212 has a third volume-averaged refractive index, wherethe first volume-averaged refractive index is greater than both thesecond volume-averaged refractive index and the third volume-averagedrefractive index. Because of the differences in the indices ofrefraction, the lower and upper passive waveguide cladding layers 212,216 help provide optical confinement within the passive waveguide core214.

While some examples of layer compositions for the passive waveguideheterostructure 210 have been described, it will be understood thatother materials may be equally used without departing from the scope ofthe present disclosure. For example, in some embodiments, the passivewaveguide heterostructure 210 may alternatively include InGaAsP,InGaAs/AlInAs multi-quantum wells, or an InGaAs/AlInAs superlattice.Further, in various embodiments, the terms “passive waveguide” or“passive waveguide heterostructure” may include any of a plurality ofmaterial systems configured to guide light along a particular direction,for example, by utilizing a waveguide core layer surrounded by claddinglayers having a lower index of refraction than the waveguide core layer.

After formation of the passive waveguide heterostructure 210, a QCLheterostructure 218 is formed over the passive waveguide heterostructure210. The QCL heterostructure 218 may include a lower cladding layer 220formed over the upper passive waveguide cladding layer 216 (of thepassive waveguide heterostructure 210), a QCL core layer 240 formed overthe lower cladding layer 220, and an upper cladding layer 224 formedover the QCL core layer 240. In some embodiments, the QCL core layer 240may include a lower confinement layer 302, a QCL active region 222 thatprovides QCL gain, and an upper confinement layer 304. FIG. 3 provides across-sectional view of the device 200 along a section B-B′ shown inFIG. 2A, illustrating an alternative view of the QCL core layer 240 andthe lower and upper confinement layer 302, 304. In various embodiments,the lower confinement layer 302 and the upper confinement layer 304provide high refractive index layers around the QCL active region 222,which are key to enabling efficient lasing operation of the device 200.

In some embodiments, the lower cladding layer 220 includes an InP lowercladding layer doped to 2×10¹⁶ cm⁻³ (e.g., with Si). In at least oneexample, the lower cladding layer 220 has a thickness of about 300 nmand a refractive index of about 3.08. In some embodiments, the lowercladding layer 220 has a thickness in a range of about 200-400 nm. Thelower confinement layer 302 may include an In_(0.53)Ga_(0.47)As layer.In some embodiments, the lower confinement layer 302 has a thickness ofabout 350 nm and a refractive index of about 3.38. In some embodiments,the lower confinement layer 302 has a thickness in a range of about150-400 nm. In some examples, the lower confinement layer 302 may bedoped to about 2×10¹⁶ cm⁻³ (e.g., with Si). In various embodiments, theQCL active region 222 is composed of a plurality (e.g., 30) ofstrain-compensated InGaAs/AlInAs QCL stage layers configured to providean emission wavelength of about 4.6 μm. The QCL active region 222 mayalso be doped to an average doping level of 1.8×10¹⁶ cm⁻³. In anexample, the QCL active region 222 has a thickness of about 1660 nm anda refractive index of about 3.26. In some embodiments, the QCL activeregion 222 has a thickness in a range of about 1200-1800 nm. In somecases, the upper confinement layer 304 may include anIn_(0.53)Ga_(0.47)As layer. In some embodiments, the upper confinementlayer 304 has a thickness of about 500 nm and a refractive index ofabout 3.38. In some embodiments, the upper confinement layer 304 has athickness in a range of about 250-600 nm. Thus, in some examples, thelower confinement layer 302 and the upper confinement layer 304 may havesubstantially the same refractive index, and a larger refractive indexthan the QCL active region 222. In another example, the upperconfinement layer 304 has a thickness of about 350 nm. In some examples,the upper confinement layer 304 may be doped to about 2×10¹⁶ cm⁻³ (e.g.,with Si). In some examples, the upper cladding layer 224 includes an InPupper cladding layer doped to 5×10¹⁶ cm⁻³ (e.g., with Si). In at leastone example, the upper cladding layer 224 has a thickness of about 2700nm and a refractive index of about 3.08.

In some cases, and as shown in device 400 of FIG. 4, a current injectionlayer 402 may optionally be formed between the upper passive waveguidecladding layer 216 and the lower cladding layer 220. In someembodiments, the current injection layer includes In_(0.53)Ga_(0.47)As.In at least one example, the current injection layer 402 may have athickness of about 200 nm and may be doped to about 1×10¹⁸ cm⁻³ (e.g.,with Si). In some embodiments, the current injection layer 402 has athickness in a range of about 100-300 nm. For each of the devices 200and 400, and in some embodiments, an InP outer cladding layer may beformed over the upper cladding layer 224, and a contact layer includingInP or In_(0.53)Ga_(0.47)As may be formed over the outer cladding layer,as shown in the example of FIGS. 10A and 10B. In an example, the InPouter cladding layer has a thickness of about 150 nm and is doped toabout 2×10¹⁷ cm⁻³ (e.g., with Si). In some embodiments, the InP outercladding layer has a thickness in a range of about 150-2000 nm. In someembodiments, the InP contact layer may have a thickness of about 200 nmand may be doped to about 5×10¹⁸ cm⁻³ (e.g., with Si). While someexamples of layer compositions and thicknesses for each of the passivewaveguide heterostructure 210, the QCL heterostructure 218, the optionalcurrent injection layer 402, and the outer cladding and contact layershave been described, those of skill in the art in possession of thepresent disclosure will recognize other layer compositions andthicknesses which may also be used without departing from the scope ofthis disclosure.

After growth of the material layers used to form the passive waveguideheterostructure 210 and the QCL heterostructure 218, fabrication stepsfor the devices 200, 400 may be substantially similar to those used forconventional ridge waveguide QCLs. For example, a patterned siliconnitride hard mask may be formed over the grown layers and used to definethe QCL gain section 202 and the taper section 204 using a dry etchingprocess through the patterned hard mask. In an embodiment, QCL ridgeetching may be stopped at the middle of the lower cladding layer 220. Insome cases, another patterned hard mask may be formed and used to definethe passive waveguide section 206 using another dry etching processthrough the patterned hard mask. In an embodiment, waveguide ridgeetching may be stopped 200 nm below the In_(0.53)Ga_(0.47)As passivewaveguide core layer 214. The sidewalls of the QCL gain section 202 maybe insulated using a 400-nm-thick layer of silicon nitride. Thereafter,in some embodiments, top and bottom contacts may be formed by e-beamevaporation of Ti/Au (20 nm/400 nm) layers followed by a lift-offprocess. In embodiments including the optional current injection layer402, a Ti/Au side contact may also be formed. In some cases, theremaining silicon nitride layer on top of the passive waveguide may beremoved by a reactive ion etching process.

With respect to device operation and referring again to the example ofFIGS. 2A and 2B, mid-IR radiation may be generated in the QCL gainsection 202, upon appropriate biasing of the device 200, and acorresponding laser mode 230 propagates into the taper section 204 wherethe laser mode is coupled to a passive waveguide mode 232 of the passivewaveguide section 206. Generally, the highest output power at facets ofthe waveguide section 206 may be achieved by maximizing modal gain inthe QCL gain section 202 and coupling efficiency in the taper section204. In some cases, such a process may in some cases involve iterativeoptimization of the QCL heterostructure 218, the passive waveguideheterostructure 210, and the taper dimensions of the taper section 204.It is also noted that in some respects, the devices disclosed herein(e.g., such as the devices 200, 400) may be viewed as double-waveguideheterostructures including a first waveguide core (e.g., the QCL activeregion 222) and a second waveguide core (e.g., the undopedIn_(0.53)Ga_(0.47)As passive waveguide core layer 214) that arevertically separated from each other by one or more cladding layers(e.g., such as the upper passive waveguide cladding layer 216 and thelower cladding layer 220), as discussed above.

In order to achieve low-threshold operation of the QCLs on the III-v PICplatform disclosed herein at or near room temperature, as well asefficient optical power coupling from the QCL to the passive waveguides,a careful balance between the thicknesses of the QCL active region 222and the passive In_(0.53)Ga_(0.47)As waveguide core 214 should beprovided. In particular, because the refractive index of the undopedIn_(0.53)Ga_(0.47)As waveguide core 214 (about 3.38 at the 4.6 μmemission wavelength of the QCL) is larger than that of the QCL activeregion 222 (about 3.26 at the 4.6 μm emission wavelength of the QCL), achoice of a thick passive waveguide core 214 layer of the passiveIn_(0.53)Ga_(0.47)As waveguide can cause significant modal leakage inthe QCL gain section 202, which can lead to high threshold currentdensity of the laser or prevent laser operation. On the other hand,making the passive waveguide core 214 thickness too thin reduces thetaper coupling efficiency (e.g., in the taper section 204) as theeffective refractive indexes of the QCL and waveguide modes become toodifferent from one another. In some embodiments, and to keep the lasermode confined in the QCL gain section 202, the effective refractiveindex of an InGaAs waveguide mode below the QCL gain section 202 shouldbe lower than that of a QCL mode in the QCL gain section 202. Thepassive waveguide cladding layer thicknesses between the QCL activeregion 222 and the passive InGaAs waveguide core 214 should also bechosen to provide both high QCL mode confinement in the QCL gain section202 and high coupling efficiency in the taper section 204. Further, andas noted above, providing the higher refractive index layer (the lowerconfinement layer 302 and the upper confinement layer 304) around theQCL active region 222 enable efficient lasing operation of the device200.

Elaborating on mode confinement, and with reference to FIG. 5A,illustrated therein is a plot 500 of mode confinement factor (%) in theQCL active region 222 (square symbols) and in the passive waveguide core214 (circular symbols) for different values of the In_(0.53)Ga_(0.47)Aspassive waveguide core 214 thickness. The results are shown for a TM₀₀mode and are obtained using COMSOL® simulations. For purposes of thesimulation data of FIGS. 5A-5D, the composition and thickness of alldevice layers, except for the passive waveguide core 214, are assumed tobe the same. Various other embodiments of the layer compositions andthicknesses of the device 200 have been previously described above. Asshown in FIG. 5A, the QCL active region 222 confinement is maintained ataround 60% until the passive waveguide core 214 thickness exceeds about850 nm, and above that thickness the QCL active region 222 confinementquickly drops. An opposite trend is observed with the fundamental TM₀₀mode confinement in the In_(0.53)Ga_(0.47)As passive waveguide core 214.Based on the data of FIG. 5A, it is evident that a thickness of about750 nm for the passive waveguide core 214 (as discussed above inreference to the device 200) ensures high mode confinement in the QCLactive region 222 of the QCL gain section 202. In some embodiments, andin accordance with the data of FIG. 5A, the passive waveguide core 214may have a thickness of between 750-850 nm, while maintaining high modeconfinement in the QCL active region 222. FIGS. 5B, 5C, and 5Dillustrate cross-sectional profiles of the simulated fundamental modefor passive waveguide core 214 thicknesses of 750 nm, 1000 nm, and 1200nm, respectively, which further underscore the data of FIG. 5A. In someembodiments, FIGS. 5B, 5C, and 5D provide the cross-sectional profilealong a section C-C′ of FIG. 2B.

In various embodiments of the QCL core layer 240, the lower confinementlayer 302 and the upper confinement layer 304 disposed beneath and abovethe QCL active region 222 of the disclosed devices may also serve toimprove mode confinement in the QCL active region 222, as illustrated inthe examples of FIGS. 6A, 6B, 7A, and 7B. FIGS. 6A and 7A providecross-sectional views along a section substantially similar to sectionB-B′ of FIG. 2A. In particular, FIG. 6A shows a device 600 including thepassive waveguide heterostructure 210 and the QCL heterostructure 218,similar to the structures described above, but not including the lowerconfinement layer 302 or the upper confinement layer 304 in the QCL corelayer 240. In this case, the QCL core layer 240 only includes the QCLactive region 222. FIG. 7A shows a device 700, substantially similar tothe device 200 of FIG. 3, and including the passive waveguideheterostructure 210, the QCL heterostructure 218, and the lowerconfinement layer 302 and the upper confinement layer 304 around the QCLactive region 222 in the QCL core layer 240.

FIG. 6B shows a cross-sectional profile of the simulated modeconfinement, corresponding to the device 600 of FIG. 6A, within the QCLgain section 202 and along a section substantially similar to thesection C-C′ of FIG. 2B. FIG. 7B shows a cross-sectional profile of thesimulated mode confinement, corresponding to the device 700 of FIG. 7A,within the QCL gain section 202 and also along a section substantiallysimilar to the section C-C′ of FIG. 2B. For the device 600, notincluding the lower confinement layer 302 or the upper confinement layer304, the mode confinement factor in the QCL active region 222 may beequal to about 16.82% and the mode confinement factor in the passivewaveguide core 214 may be equal to about 34.11%, which indicatessignificant modal leakage in the QCL gain section 202. For the device700, which includes the lower confinement layer 302 and the upperconfinement layer 304, the mode confinement factor in the QCL activeregion 222 may be equal to about 65.07% and the mode confinement factorin the passive waveguide core 214 may be equal to about 0.65%. Thus, theconfinement layers 302, 304 may be used to effectively mitigate modalleakage, and ensure high mode confinement, in the QCL active region 222of the QCL gain section 202.

With respect to mode coupling, mode coupling (e.g., in the taper section204) from the QCL gain section 202 to the In_(0.53)Ga_(0.47)As passivewaveguide section 206 occurs when the effective refractive indices ofthe modes confined in the two sections (the QCL gain section 202 and thepassive waveguide section 206) are substantially matched. In someembodiments, increasing the interaction length (e.g., within the tapersection 204) for coupling near the index matching point may provideenhanced mode coupling efficiency. In some examples, to find the QCLridge width that provides the modal refractive index matching to themode in the passive In_(0.53)Ga_(0.47)As waveguide, separate QCL andIn_(0.53)Ga_(0.47)As waveguide heterostructures having different ridgewidths may be simulated and analyzed. The simulation results may providereference waveguide widths to initiate and optimize the taper design(e.g., of both the QCL heterostructure and the passive waveguideheterostructure). With reference to FIG. 8, illustrated therein is aplot 800 showing effective refractive indices of the QCL waveguide modes(solid lines) and the passive In_(0.53)Ga_(0.47)As waveguide modes(dashed lines) as a function of ridge width (e.g., QCL ridge width andpassive waveguide ridge width). In particular, the plot 800 shows theeffective indices of the TM₀₀ mode in the QCL ridge waveguide and of theTM₀₀ mode in the passive In_(0.53)Ga_(0.47)As ridge waveguide. As shown,when the QCL ridge width is about 2.5 μm and the passive waveguide ridgewidth is about 5 μm, the effective indices of the fundamental TM₀₀ modesof the two waveguides are found to be matching, as indicated by line802. In various embodiments and given the aim to achieve a singlespatial mode operation of the PIC device 200, both waveguides (e.g., ofthe QCL heterostructure and the passive waveguide heterostructure)should be designed to confine fundamental modes. In addition, and aspreviously discussed, tapering of both the QCL heterostructure and thepassive waveguide heterostructure may be performed (as shown in FIG.2A). As such, and with consideration of the data of FIG. 8, the taper ofeach of the QCL ridge width and the passive waveguide ridge width may insome embodiments be designed to increase the length (within the tapersection 204) along which the effective indices of the fundamental TM₀₀modes of the two waveguides are found to be matching.

Referring now to FIGS. 9A, 9B, 9C, and 9D, illustrated therein aresimulation results for a transverse mode (TM) transition (FIG. 9A) of aTM₀₀ QCL mode to a TM₀₀ waveguide mode along the section A-A′ shown inFIG. 2A, as well as the cross-sectional profiles of the simulated modeconfinement at each of the QCL gain section 202 (FIG. 9B), the tapersection 204 (FIG. 9C), and the passive waveguide section 206 (FIG. 9D).In particular, FIG. 9B provides the cross-sectional profile along asection C-C′ of FIG. 2B, FIG. 9C provides the cross-sectional profilealong a section D-D′ of FIG. 2B, and FIG. 9D provides thecross-sectional profile along a section E-E′ of FIG. 2B. Thethree-dimensional eigenmode simulations of FIGS. 9A-9D were generatedusing a finite element method simulation tool (MODE Solutions developedby Lumerical Inc. of Vancouver, BC, Canada) to estimate the tapercoupling efficiency, which is defined as the ratio of optical power inthe waveguide section 206 to optical power in the QCL gain section 202of the device 200.

As previously discussed, the taper design of the taper section 204 usestwo tapering steps, the first taper step 203 and the second taper step205, as shown in FIG. 2A. The first taper step 203 quickly decreases theeffective refractive index of the QCL mode to a value that is close tothat of the passive waveguide mode, while the second taper step 205provides a relatively large interaction length near the index matchingpoint, discussed above, for efficient adiabatic mode transition withinthe taper section 204. In some embodiments, the taper shape (e.g., ofthe QCL heterostructure in the taper section 204) may be expressed as:

${{W_{hf}(x)} = {W_{{hf} - e} + {\left( {W_{{hf} - i} - W_{{hf} - e}} \right)\left( {1 - \frac{x}{L}} \right)^{\alpha}}}},$

where x is the propagation direction, W_(hf) is the half width of thetaper at x, W_(hf−1), and W_(hf−e) are the initial and end half width ofthe taper, respectively, L is the taper length, and a is the nonlinearfactor. In an embodiment of the device 200 including a QCL ridge widthof 6 μm (e.g., the first QCL ridge width ‘W1 _(QCL)’ of FIG. 2A), thetwo-step linear (α=1) taper may include the first linear taper step 203having a length of about 0.1 mm with the QCL ridge width changing from 6μm to 5 μm (e.g., the second QCL ridge width ‘W2 _(QCL)’ of FIG. 2A),and the second linear taper step 205 having a length of about 0.9 mmwith the QCL ridge width changing from 5 μm to 1.75 μm (e.g., the thirdQCL ridge width ‘W3 _(QCL)’ of FIG. 2A). The three-dimensional eigenmodesimulation shown in FIG. 9A, where the simulation is performed utilizingthe above taper geometry and layer thicknesses and compositionsdescribed above, indicates that such a taper may provide a couplingefficiency of about 72% for the device 200. In some cases, higherefficiency may be achieved by increasing the thickness of the passivewaveguide layer 214 and further optimizing the taper dimensions. In atleast one embodiment, the adiabatic taper of the passive waveguideheterostructure 210, within the taper section 204, is about 1 mm longand has a width that tapers from about 30 microns to about 5 microns.

Devices (e.g., such as the devices 200, 400, 700) fabricated inaccordance to the various embodiments discussed above may be mountedepitaxial-side-up (epi-up) on copper blocks using indium solder forelectrical and optical characterizations. With reference to FIGS. 10Aand 10B, illustrated therein is a device 1000 and a device 1002,respectively, which show biasing configurations for devicecharacterization. The devices 1000, 1002 may in various aspects besubstantially similar to the devices 200, 400, 700 discussed above. Forexample, each of the devices 1000, 1002 includes the passive waveguideheterostructure 210, the QCL heterostructure 218, the QCL core layer240, as well as other layers previously described. The devices 1000,1002 are further shown to include the optional current injection layer402. However, other embodiments may not include the current injectionlayer 402. The devices 1000, 1002 also illustrate a silicon nitridelayer 1004, which as discussed above, may be used to insulate sidewallsof the QCL gain section 202. In some embodiments, the devices 1000, 1002may also include an outer cladding layer and a contact layer, asdiscussed above, and illustrated as layer 1011. In addition, each of thedevices 1000, 1002 may include one or more metal layers 1006 (e.g., suchas Ti/Au) that may be used to form a top contact 1008, a bottom contact1010, and a side contact 1012 (for embodiments including the optionalcurrent injection layer 402).

During operation, the devices 1000, 1002 are negatively biased at thetop contact 1008, while current is injected either through the substrate(via the bottom contact 1010) or through the current injection layer (ifpresent, via the side contact 1012). For purposes of this example,fabricated devices were tested in pulsed mode with 50-ns current pulsesat 50 kHz repetition frequency. Lasing spectra were measured using aFourier-transform infrared spectrometer equipped with adeuterate-triglycine sulfate detector and a potassium bromide beamsplitter. Output power of the devices was collected with a 7-mm-diameterpolished metal pipe and directed to a calibrated thermopile detector. Inthe present example, 100% power collection efficiency in the measurementsetup is assumed. All measurements were performed at room temperature.

Referring now to FIGS. 11A, 11B, and 11C, illustrated therein aremeasurement results for a reference device (which includes only the QCLgain section 202 with cleaved facets) and a waveguide-coupled device(including the QCL gain section 202, the taper section 204, and thewaveguide section 206, such as shown and described with reference to thedevice 200). In the present example, the reference device has a cavitylength (e.g., QCL gain section 202 length) of 3.75-mm and a QCL ridgewidth of 4.3 μm (W1 _(QCL)). The waveguide-coupled devices have a QCLridge width of 4.3 μm (W1 _(QCL)), a QCL gain section 202 length of 3.75mm, a 1-mm-long taper section 204, and 5-μm-wide passive waveguides(e.g., passive waveguide ridge width, W2 _(PW)) of different lengths.FIG. 11A shows a first measured spectrum 1102 of the reference deviceand a second measured spectrum 1104 of the waveguide-coupled device. Thewaveguide-coupled device generating the spectrum 1104 has a passivewaveguide length of 6.5 mm. The devices were observed to emit near 4.6μm wavelength, as designed. FIG. 11B shows light-current characteristicsof the reference device (1106) and waveguide-coupled devices havingpassive waveguides of different lengths: 6.5 mm (1108), 9.5 mm (1110),and 11 mm (1112). Output power of the waveguide-coupled QCLs devices wasmeasured from the passive waveguide facets. The output power for each ofthe waveguide-coupled QCL devices at the roll over (peak) points are 110mW, 103 mW, and 89 mW for the devices with the waveguide lengths of 6.5mm (1108), 9.5 mm (1110), and 11 mm (1112), respectively, while thereference device (1106) has a roll over power of 262 mW.

It is noted that undercut during dry etching may impact as-fabricateddevice dimensions. In the present case, undercut during dry etchingresults in the 1.7 μm narrower-than-designed width of the QCL and tapersections (e.g., QCL ridge width of 4.3 μm instead of 6 μm, as discussedabove). The variation of the dimension due to such undercutting mayreduce the coupling efficiency of the taper section 204 from about 72%(estimated by the eigenmode simulation, previously discussed) to about50%. By mitigating such issues, and fabricating QCL and taper sectionswith as-designed dimensions (e.g., as-designed widths), experimentaldevice performance is expected to significantly improve.

The current density shown in FIG. 11B may be estimated by dividing biascurrent by the area of the QCL gain section 202 (e.g., ridgewidth×cavity length) which is identical for the reference device and thewaveguide-coupled devices, assuming current spreading to the tapersection is negligible. In the present example, the threshold currentdensities (J_(th)) of the waveguide-coupled devices increase as thepassive waveguide length increases. About a 10% increase in J_(th) isobserved for the 11-mm-long (1112) waveguide-coupled device, compared toJ_(th) of the reference (1106) device (4.86 kA/cm²). As shown in FIG.11B, both the reference and waveguide-coupled devices reach a maximumpower output at a pump current density of approximately 9.4 kA/cm²,indicating that the current spreading from the QCL gain section 202 tothe taper section 204 in the waveguide-coupled devices is indeednegligible.

Similar power output (FIG. 11B) of the waveguide-coupled devices withdifferent passive waveguide lengths (6.5 mm, 9.5 mm, and 11 mm) confirmslow optical loss in the passive waveguides (e.g., in the passivewaveguide section 206). The power output of the waveguide-coupled devicewith the 11-mm-long (1112) waveguide section was only 20% lower thanthat of the device with 6.5-mm-long (1108) passive waveguide. Assumingthat such power reduction originates from the waveguide lengthdifference, it may be estimated that the passive waveguide loss fordevices fabricated in accordance with the various embodiments disclosedherein is about ˜2.2 dB/cm.

One of the advantages of the III-V homogeneous photonic integrationapproach, described herein, is that the lasers may be biased through thedoped substrate similar to conventional QCLs. With reference to FIG.11C, illustrated therein are voltage-current characteristics for thesame reference device measured according to different biasconfigurations (shown in FIGS. 10A and 10B). For example, a curve 1114shows the voltage-current characteristics for the reference devicebiased according to a top-bottom bias configuration (FIG. 10B), and acurve 1116 shows the voltage-current characteristics for the samereference device biased according to a top-side-bottom biasconfiguration (FIG. 10A). In the present example, the device with thetop-bottom configuration (curve 1114) produced an additional seriesresistance of approximately 1.33Ω, compared to the device with bothbottom and lateral current injection (curve 1116). It is noted that theresistance of the lateral current injection layer 402 is estimated to beless than 0.1Ω, much smaller than the resistance of the undoped passivewaveguide layers in the top-bottom biased device (curve 1114). Thus, theseries resistance of the top-side-bottom biased device (curve 1116) isprincipally determined by the small (<0.005Ω) resistance of the currentinjection layer 402. In some embodiments, and assuming that thebackground doping level of the undoped passive waveguide layers is about3×10¹⁵ cm⁻³, the resistivity values of In_(0..53)Ga_(0.47)As and InP maybe estimated (e.g., from FIG. 1B) to be about 0.22 Ω·cm and 0.463 Ω·cm,respectively. In an example, assuming the thicknesses (t) of the undopedIn_(0.53)Ga_(0.47)As and InP passive waveguide layers to be,respectively, 0.75 μm and 4.5 μm, and assuming a cross-sectional area(S) of the current path to be 4.3 μm×3.75 mm (QCL ridge width×QCL gainsection length), a calculated resistance (R=ρt/S) may be determined tobe about 1.39Ω. In the present example, the estimated voltage across thepassive layers at the rollover point (1=1.5 A, FIG. 11B) is therefore2.085 V, which is in good agreement with experimental results (e.g., asshown in FIG. 11C). The measurements thus indicate that III-V MWIR QCLPICs, as disclosed herein, may be operated using current extractionthrough the substrate without introducing a heavily-doped currentinjection layer.

The measurement results for the PIC QCLs devices discussed above withreference to FIGS. 11A, 11B, and 11C demonstrate a significant gain inoptical power measured from the passive waveguide facet, for example, ascompared to at least some existing devices. To be sure, even highervalues of optical power may be coupled to the passive waveguides of thesame dimensions, for instance, if the ridge width of QCL gain section isincreased and if a high-reflection coating (e.g., such asAl₂O₃/Ti/Au/=100 nm/10 nm/100 nm) is applied to the QCL cleaved facet.Referring to FIG. 12, illustrated therein is a plot 1200 showing thepeak output optical power collected from a QCL PIC with a 6.5-mm-long90-degree bent passive waveguide coupled to a 1-mm-long taper sectionand a 10.5-μm-wide, 3.75-mm-long QCL gain section. In this example, thepassive waveguide has the same dimensions (e.g., such as passivewaveguide ridge width) as for the devices described above. In addition,in an embodiment, a relatively large waveguide bending radius of about250 μm may be used to ensure a negligibly small bending loss (e.g., <0.1dB). As shown in FIG. 12, the wider QCL gain section andhigh-reflectivity coating applied to the QCL facet result in a reducedthreshold current density of 3.5 kA/cm² and higher slope efficiency,compared to PIC devices with the 4.3-μm wide gain sections, previouslydescribed. In the present example, nearly 290 mW of peak power outputfrom the waveguide facet was measured, which represents approximately anorder of magnitude improvement over the power outputs obtained fromheterogeneously-integrated QCLs. Given the estimated passive waveguidefacet reflectivity of about 30%, it is determined that over 400 mW ofmid-IR optical power at λ≈4.6 μm is coupled from the QCL gain section202 to the passive waveguide section 206 for the present device.

The III-V homogeneous photonic integration approach, described herein,further provides for the fabrication of buried-heterostructure (BH)QCL-based PIC devices, such as a BH QCL-based PIC device 1300 shown inFIGS. 13A and 13B. In various examples, BH QCL-based PIC devices areconfigured to provide low-waveguide loss and improved heat extraction ofthe QCL gain section 202. The BH QCL-based PIC device 1300 of FIGS. 13Aand 13B are in various respects substantially similar to the device 200as illustrated in FIGS. 2A and 3, respectively. Specifically, FIG. 13Ashows a top-down view of the BH QCL-based PIC device 1300, and FIG. 13Bshows a cross-sectional view of the BH QCL-based PIC device 1300 along asection F-F′ of FIG. 13A. In some embodiments, the BH QCL-based PICdevice 1300 includes the passive waveguide heterostructure 210, the QCLheterostructure 218, the QCL core layer 240, as well as other layerspreviously described. However, by way of example, fabrication of the BHQCL-based PIC device 1300 may further include epitaxial overgrowth ofFe-doped InP cladding layers over a ridge waveguide QCL devices, such asthose discussed herein. In some embodiments, a dielectric mask (e.g.,such as silicon nitride) may be deposited over the BH QCL-based PICdevice 1300 and patterned (e.g., by a photolithography and etchingprocess) to expose regions on either side of the QCL heterostructure 218(including exposing opposing sidewalls of the QCL heterostructure 218)within the QCL gain section 202, while the dielectric mask remains overa top surface QCL heterostructure 218 and over the taper section 204 andthe passive waveguide section 206. Thereafter, in some embodiments, anFe-doped InP cladding layer 1302 may be epitaxially grown, by one of themethods discussed above, over the exposed regions on either side of theQCL heterostructure 218 (including over the exposed opposing sidewallsof the QCL heterostructure 218) within the QCL gain section 202, whileother portions of the BH QCL-based PIC device 1300 remain protected bythe patterned dielectric mask. After formation of the Fe-doped InPcladding layer 1302, in some cases, the patterned dielectric mask isremoved (e.g., by an etching process). In alternative embodiments, theFe-doped InP cladding layer 1302 may be formed over the entire BHQCL-based PIC device 1300. In some embodiments, the overgrown Fe-dopedInP cladding layer 1302 may be partially removed to retain desiredfunctionalities of the photonic devices integrated therein. It is alsonoted that implementation of the BH process to QCL-based PIC devicesfabricated using a heterogeneous integration approach may be highlychallenging due to the presence of a physical bonding interface,previously described, which is problematic for high-temperatureepitaxial processes.

With respect to the description provided herein, the present disclosureprovides a homogeneously-integrated III-V mid-IR QCL-based PIC. Unlikemid-IR PICs based on heterogeneous integration of QCLs with Si-basedpassive waveguiding platforms, embodiments of the present disclosureprovide for a single epitaxial growth of both the QCL and the passivewaveguide heterostructures. As a result, the reliability, heatextraction, and performance of the mid-IR PIC sources is dramaticallyimproved. In some examples, the disclosed QCL PIC devices provide 290 mWof optical power measured from the facet of the passiveIn_(0.53)Ga_(0.47)As optical waveguides at room temperature. Such anoutput power represents an order of magnitude improvement over the bestresults obtained with at least some existing heterogeneously-integrateddevices. In some embodiments, the disclosed devices exhibit a passivewaveguide loss of about ˜2.2 dB/cm. The mid-IR PICs disclosed hereininclude a QCL gain section, a taper section, and a passive waveguidesection, where the taper section includes a two-step adiabatic taper tocouple the QCL mode to the passive waveguide mode. In variousembodiments, a lower confinement layer and an upper confinement layermay also be formed to provide high refractive index layers above andbelow the QCL active region of the QCL gain section, which improve lasermode confinement and provide for efficient lasing operation. In someembodiments, the adiabatic taper coupling efficiency is estimated to beabout 72%, although higher efficiency may be achieved by adjusting thethickness of the device heterostructures and further optimizing thetaper dimensions. In some examples, homogeneously-integrated MWIR QCLPICs may be operated using current extraction through the substrate,rather than using lateral current extraction. Such an approach makesfabrication, packaging, and operation of QCL gain elements in the PICsimilar to that of conventional QCLs, which is crucial for thedevelopment of reliable mid-IR QCLs PICs for many applications demandingcontinuous-wave and high-power operation.

Those of skill in the art will readily appreciate that the methods andstructures described herein may be applied to a variety of othersemiconductor devices to advantageously achieve similar benefits fromsuch other devices without departing from the scope of the presentdisclosure. Further, in some examples, the devices disclosed herein maybe integrated with and/or include various other devices and features,such as transistors, resistors, capacitors, inductors, diodes, fuses,memory and/or other logic circuits, etc. In some embodiments, thedevices disclosed herein may include a plurality of interconnected PICs.In some examples, the QCL PIC devices described herein may further beintegrated on a substrate including other types of optoelectronicdevices such as resonator sensors, detectors, modulators, couplers,isolators, photodiodes, or other appropriate device. In some cases, theQCL PIC devices may be formed over and coupled (e.g., by way of one ormore vias) to underlying CMOS circuits and/or devices, for example, aspart of a 3D hybrid integrated photonics/CMOS device.

Thus, one of the embodiments of the present disclosure described asemiconductor device including a substrate, a passive waveguideheterostructure formed over the substrate, and a quantum cascade laser(QCL) heterostructure formed over the passive waveguide heterostructure.In some embodiments, the passive waveguide heterostructure includes alower passive waveguide cladding layer disposed over the substrate, apassive waveguide core layer disposed over the lower passive waveguidecladding layer, and an upper passive waveguide cladding layer disposedover the passive waveguide core layer. By way of example, the QCLheterostructure includes a lower cladding layer disposed over the upperpassive waveguide cladding layer, a QCL core layer disposed over thelower cladding layer, and an upper cladding layer disposed over the QCLcore layer. In some embodiments, the QCL core layer includes a lowerconfinement layer, a QCL active region disposed over the lowerconfinement layer, and an upper confinement layer disposed over the QCLactive region. In various cases, the QCL active region has a firstrefractive index, the lower confinement layer has a second refractiveindex, and the upper confinement layer has a third refractive index. Insome embodiments, the second refractive index and the third refractiveindex are both greater than the first refractive index.

In another of the embodiments, discussed is a photonic integratedcircuit device including a passive waveguide section formed over asubstrate, a quantum cascade laser (QCL) gain section formed over thesubstrate and adjacent to the passive waveguide section, and a tapersection disposed between and in contact with each of the passivewaveguide section and the QCL gain section. In some embodiments, thepassive waveguide section includes a passive waveguide core layerdisposed between a first cladding layer and a second cladding layer. Insome examples, the QCL gain section includes a QCL active regiondisposed between a first confinement layer and a second confinementlayer, where the QCL active region has a lower index of refraction thaneach of the first and second confinement layers. In some embodiments,the taper section is configured to optically couple the QCL gain sectionto the passive waveguide section.

In yet another of the embodiments, discussed is a semiconductor deviceincluding a taper section. The semiconductor device further includes apassive waveguide section adjacent to and in contact with a first sideof the taper section, where the passive waveguide section includes apassive waveguide heterostructure having a passive waveguide core layerdisposed between a first cladding layer and a second cladding layer. Thesemiconductor device further includes a quantum cascade layer (QCL) gainsection adjacent to and in contact with a second side of the tapersection opposite the first side of the taper section, where the QCL gainsection includes a QCL active region disposed between a lowerconfinement layer and an upper confinement layer, and where the QCLactive region has a lower index of refraction than either the lowerconfinement layer or the upper confinement layer. In some embodiments,the semiconductor device further includes an epitaxial cladding layerdisposed on either side of the QCL heterostructure within the QCL gainsection, where the epitaxial cladding layer covers opposing sidewalls ofthe QCL heterostructure. In various embodiments, the taper section isconfigured to optically couple the QCL gain section to the passivewaveguide section.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a passive waveguide heterostructure formed over the substrate, whereinthe passive waveguide heterostructure includes a lower passive waveguidecladding layer disposed over the substrate, a passive waveguide corelayer disposed over the lower passive waveguide cladding layer, and anupper passive waveguide cladding layer disposed over the passivewaveguide core layer; and a quantum cascade laser (QCL) heterostructureformed over the passive waveguide heterostructure, wherein the QCLheterostructure includes a lower cladding layer disposed over the upperpassive waveguide cladding layer, a QCL core layer disposed over thelower cladding layer, and an upper cladding layer disposed over the QCLcore layer; wherein the QCL core layer includes a lower confinementlayer, a QCL active region disposed over the lower confinement layer,and an upper confinement layer disposed over the QCL active region; andwherein the QCL active region has a first refractive index, the lowerconfinement layer has a second refractive index, and the upperconfinement layer has a third refractive index, and wherein the secondrefractive index and the third refractive index are both greater thanthe first refractive index.
 2. The semiconductor device of claim 1,wherein the passive waveguide core layer has a first volume-averagedrefractive index, wherein the upper passive waveguide cladding layer hasa second volume-averaged refractive index, wherein the lower passivewaveguide cladding layer has a third volume-averaged refractive index,and wherein the first volume-averaged refractive index is greater thanboth the second volume-averaged refractive index and the thirdvolume-averaged refractive index.
 3. The semiconductor device of claim1, wherein the lower passive waveguide cladding layer, the passivewaveguide core layer, the upper passive waveguide cladding layer, thelower cladding layer, the QCL core layer, and the upper cladding layereach include one or more epitaxially-grown semiconductor materiallayers.
 4. The semiconductor device of claim 1, further comprising: apassive waveguide section including the passive waveguideheterostructure; a QCL gain section including the QCL heterostructure,wherein the QCL gain section is disposed adjacent to the passivewaveguide section; and a taper section disposed between and in contactwith each of the passive waveguide section and the QCL gain section,wherein the taper section is configured to optically couple the QCL gainsection to the passive waveguide section.
 5. The semiconductor device ofclaim 1, wherein the substrate includes InP.
 6. The semiconductor deviceof claim 1, wherein the passive waveguide core layer of the passivewaveguide heterostructure includes undoped In_(0.53)Ga_(0.47)As, andwherein the lower passive waveguide cladding layer and the upper passivewaveguide cladding layer both include undoped InP cladding layers. 7.The semiconductor device of claim 1, wherein the passive waveguide corelayer of the passive waveguide heterostructure has a thickness of about750 nm, wherein the lower passive waveguide cladding layer has athickness of about 3 microns, wherein the upper passive waveguidecladding layer has a thickness of about 1.5 microns, and wherein thesemiconductor device is configured to operate at a wavelength of about4.6 μm.
 8. The semiconductor device of claim 1, wherein the QCL activeregion includes strain-compensated or lattice-matched epitaxialsemiconductor layers.
 9. The semiconductor device of claim 1, whereinthe QCL active region includes a strain-compensated InGaAs/AlInAsheterostructure, and wherein the lower confinement layer and the upperconfinement layer include doped In_(0.53)Ga_(0.47)As.
 10. Thesemiconductor device of claim 1, wherein the QCL active region has athickness of about 1.66 microns, wherein the lower confinement layer hasa thickness of about 350 nm, wherein the upper confinement layer has athickness of about 500 nm, and wherein the semiconductor device isconfigured to operate at a wavelength of about 4.6 μm.
 11. Thesemiconductor device of claim 1, further comprising: a current injectionlayer disposed between the upper passive waveguide cladding layer of thepassive waveguide heterostructure and the lower cladding layer of theQCL heterostructure.
 12. A photonic integrated circuit device,comprising: a passive waveguide section formed over a substrate, whereinthe passive waveguide section includes a passive waveguide core layerdisposed between a first cladding layer and a second cladding layer; anda quantum cascade laser (QCL) gain section formed over the substrate andadjacent to the passive waveguide section, wherein the QCL gain sectionincludes a QCL active region disposed between a first confinement layerand a second confinement layer, and wherein the QCL active region has alower index of refraction than each of the first and second confinementlayers; and a taper section disposed between and in contact with each ofthe passive waveguide section and the QCL gain section, wherein thetaper section is configured to optically couple the QCL gain section tothe passive waveguide section.
 13. The photonic integrated circuitdevice of claim 12, wherein the taper section is configured tosubstantially match effective refractive indices of a QCL mode and apassive waveguide mode.
 14. The photonic integrated circuit device ofclaim 12, wherein the taper section includes a two-step adiabatic taperof a QCL heterostructure and an adiabatic taper of a passive waveguideheterostructure.
 15. The photonic integrated circuit device of claim 14,wherein a first portion of the two-step adiabatic taper of the QCLheterostructure is about 0.1 mm long and has a width that tapers fromabout 6 microns to about 5 microns, and wherein a second portion of thetwo-step adiabatic taper is about 0.9 mm long and has a width thattapers from about 5 microns to about 1.75 microns.
 16. The photonicintegrated circuit device of claim 14, wherein the adiabatic taper ofthe passive waveguide heterostructure is about 1 mm long and has a widththat tapers from about 30 microns to about 5 microns.
 17. The photonicintegrated circuit device of claim 12, wherein the passive waveguidecore layer of the passive waveguide section defines a first horizontalplane, wherein the QCL active region of the QCL gain section defines asecond horizontal plane parallel to the first horizontal plane andseparated from the first horizontal plane by one or more claddinglayers.
 18. A semiconductor device, comprising: a taper section; apassive waveguide section adjacent to and in contact with a first sideof the taper section, wherein the passive waveguide section includes apassive waveguide heterostructure having a passive waveguide core layerdisposed between a first cladding layer and a second cladding layer; aquantum cascade layer (QCL) gain section adjacent to and in contact witha second side of the taper section opposite the first side of the tapersection, wherein the QCL gain section includes a QCL active regiondisposed between a lower confinement layer and an upper confinementlayer, and wherein the QCL active region has a lower index of refractionthan either the lower confinement layer or the upper confinement layer;and an epitaxial cladding layer disposed on either side of the QCLheterostructure within the QCL gain section, wherein the epitaxialcladding layer covers opposing sidewalls of the QCL heterostructure;wherein the taper section is configured to optically couple the QCL gainsection to the passive waveguide section.
 19. The semiconductor deviceof claim 18, wherein the epitaxial cladding layer includes an Fe-dopedInP cladding layer.
 20. The semiconductor device of claim 18, whereinthe epitaxial cladding layer is disposed over a portion of the passivewaveguide heterostructure within the QCL gain section.